48 



PHYSICAL ASPECTS OF HEARING 



which finally transmit the vibrations, and the associated pressures, to 

 the cochlea fluid filling the canals. 



There are two kinds of resonance theories possible at this point: one 

 based on resonances in the fluid, the other on resonances in the mem- 

 branes. It can be shown that fluid resonances are physically excluded 

 for a variety of reasons. Consider the schematic representation of the 

 cochlea in Fig. 21(c). Here the cochlea has been shown as a straight 

 tube with its two parts separated by a membrane (we neglect the 

 cochlear duct here — it simply makes the dividing membrane more com- 

 plicated). The aperture (helicotrema) connecting the two parts is also 

 shown. The actual cochlea would differ from the schematic one chiefly 

 in being twisted around into the snail-like shape. 



When the stapes moves in response to an impulse entering the ear, the 

 fluid is pushed. If the canals were filled with a gas or a compressible 

 liquid, waves could travel along the canal and bounce off the other end, 



(a) 



BM 



(b) 



Helicotrema 



RW 



Fig. 21. Sketch of the cochlea (a) and a cross section of the cochlea (b). 

 Part (b) shows the separation of the snail-like tube into Vestibular (V), Median 

 or Cochlear (C), and Tympanic (T) canals. The stapes insertion at the end 

 of the vestibular canal is indicated by the dotted line containing the .symbol S, 

 and the round window (RW) at the end of the tympanic canal is similarly indi- 

 cated. BM is the location of the basilar membrane. N is the auditory nerve as 

 it emerges from the cochlea. Part (c) shows a schematized version of the 

 cochlea unwound from its actual snail-like configuration. The basilar membrane 

 divides the two parts of the cochlea; the cochlear canal is omitted for simplicity. 

 The helicotrema allows passage of fluid from one part of the cochlea to the other. 



